The invention is in the field of electronic reproduction technology and is directed to a method and to an apparatus for processing color values generated by point-by-point and line-by-line, optoelectronic scanning of color originals in view of a modification of the reproduction scale and in view of a correction of image sharpness.
A point-by-point and line-by-line scanning of a color original is implemented, for example, in a flat bed color image scanner means. Given such a flat bed color image scanner means, the color original to be scanned is arranged on a planar originals carrier that moves continuously relative to an optoelectronic transducer of a scanner unit. The color original to be scanned is illuminated line-by-line with red, green and blue light in alternating fashion, and the scan light that is reflected by the color original or which passes through the color original and is modulated with the color information of the scanned lines is converted into analog color values in the scanner unit.
The scanner unit is essentially composed of a source of scan light, of a rotating filter wheel for the line-by-line separation of the white light produced by the scanner light source into red, green and blue light, and of an optoelectronic transducer, for example of a photodiode line (CCD line), having a following signal editing unit for converting the color components "red", "green" and "blue" acquired line-by-line in the color original into the color values (R, G, B) of the individual pixels in the scan lines. The filter wheel comprises three color-selective segments that have different spectral transmission characteristics for "red", "green" and "blue". The scanner unit is followed by a color value processing unit wherein the analog color values (R, G, B) are converted, for example, into digital color values (R, G, B), are processed for the following processes and are then stored or output on-line.
In the reproduction of color originals, a modification of the reproduction scale in comparison to the scanned color original is often undertaken, for which purpose a multitude of time-consuming calculating operations on the basis of the color values are required in the color value processing.
At the same time, image sharpness corrections (contrast correction) must often be implemented on the basis of an electronic unsharp masking. The contrast, particularly in fine details, is already diminished by comparison to the original in the production of the color original due to unsharpness in film layers as well as due to enlargement and transfer copying. Added thereto is that the resolution of the optoelectronic scanner element of a color scanner is limited due to stray light and unsharpness of the scanner objective, as a result whereof an additional reduction in contrast occurs in the reduction, the observer's eye interpreting this as unsharpness. There is therefore the necessity of restoring the reduced contrast or, respectively, the reduced image sharpness in the processing of the color values, or to even enhance them in comparison to the original for editorial reasons.
In the image sharpness correction on the basis of electronic unsahrp masking, a surrounding field value is first calculated for every current pixel from the color values of a field surrounding the current pixel, the difference between surrounding field value and color value of the current pixel is formed, and the differential value is added to the color value of the current pixel to a selectable degree as a sharpness correction value. Further time consuming calculating operations are thus required for image sharpness correction.
Due to the substantial number of color values that are to be processed in a modification of scale, there is the problem that an additional image sharpness correction could previously not be implemented in a satisfactory way without considerably exceeding the time available for the scale calculation and, thus, without disadvantageously reducing the speed in the color value processing.
Given employment of a filter wheel in the optoelectronic scanner unit, the color components "red", "green" and "blue of the color original are successively scanned line-by-line due to the continuous relative motion between the optoelectronic transducer and the color original.
Disturbing color offsets arise due to this sequential color scanning in the individual scan lines of the color original, since respectively only one color component is directly acquired for the individual pixels of a current scan line.
For improving the reproduction quality, it is already known to implement a color offset correction in that the lacking color values of the pixels of a scan line are respectively calculated from the color values of the neighboring scan lines. Further time-consuming calculating operations are required for the correction of these color offsets.